First thing's first, change the wall color to a light grey. There is too much brown in this room and I think grey will introduce a new color and brighten the space up! There are really no rules when it comes to displaying art. Check out these pictures below for some inspiration. Sometimes it looks good when there is a common theme like the same frame, or all color art, or the same matte background. Other suggestions would be to punch up the room with some colorful pillows or a rug with color!

I had a lot of art. I would divide mine into subjects, such as on one wall I put boats, seaside, ocean views. On another wall I would put art with similar colorways, on another wall I would put antique art & objects, on another wall I would put art done by the same artist & incorporate some modern objects. I would line up one side or two side of the frames, but I really don't think it matters any more.

Only one large piece for that wall.

Thanks for the paint suggestion Andrea. Would a light grey match with the couch color?